Some TMNT stuff really isn't for little kids.
In the IDW continuity, the substance known commonly as ooze is a green liquid that originates in the general region of the planet Utrominon. It was partially the overuse of ooze that ultimately rendered the planet uninhabitable to life, as its reserves had been exhausted by Utrom Emperor Quanin's ruthless expansionist policies.
Most notably, ooze is the core ingredient of mutagen, presumably being responsible for the rapid growth and occasional expanded intelligence of any animals who come into contact with it. Related substances like the psychotropic compound are also refined from the ooze.
The effects of raw ooze can be extremely varied, but typically it seems to have life-giving or healing properties. When ingested, it can cause a person's body to change into its peak condition without effort, as seen with Takeshi Tatsuo. This includes the temporary reversal of the aging process.
Additionally, the ooze was key to keeping the Utroms on Burnow Island alive during their time in stasis. The removal of the ooze had fatal effects on them, due to their inability to breathe Earth's atmosphere on their own.
The ooze has nearly limitless healing abilities, having been used to regrow Takeshi Tatsuo's lost leg, reverse the effects of John O'Neil's stroke, and allowed the almost immediate healing of Donatello's critically-injured body. This fluid was also used as part of Oroku Saki's attempt to achieve immortality, which involved mortally wounding himself before being suspended in a casket filled of ooze, which preserved him for centuries.